1. Field of the Invention
The present invention generally relates to flexible optical disks with high recording densities and a manufacturing method of the same. More particularly, a focus error due to poor following of a focus servo at a high frequency area of a tilt (surface wobble) is reduced so that recording and reproducing quality of a flexible optical disk can be greatly improved.
2. Description of the Related Art
An optical disk is removable and has a large capacity. Hence, the optical disk has been widely spread as a recoding medium for recording and reproducing. Normally, in the optical disk, a transferring layer is deposited on a polycarbonate substrate having a thickness of 1.2 or 0.6 mm, information is recorded on the transferring layer, and the information recorded on the transferring layer is reproduced. For recording or reproducing, it is necessary to condense a beam on a recording surface and therefore precision positioning of the recording surface against an optical pick up is necessary. Therefore, the substrate is made plane and has rigidity and a servo is applied to the optical pick up so that the above mentioned precision positioning is obtained.
It has been studied and researched to make an optical spot have a further smaller diameter by increasing the numerical aperture (NA) of an objective lens or by making the wavelength of a laser beam short, so that the recording capacity of the optical disk is increased. On the other hand, in order to increase the numerical aperture (NA) of the objective lens, it is necessary to make the tilt (surface wobble) of the substrate small. Therefore, it has been attempted to expand a margin of the tilt (surface wobble), by improving the plane precision of manufactured substrate, loading a tilt (surface wobble) servo on the optical pick up, or providing a thin cover layer having a thickness of an approximately 0.1 mm on the transferring layer and recording or reproducing from the side of the cover layer.
It is possible to make the tilt (surface wobble) of the optical disk substrate small by devising materials and manufacturing methods. However, this causes an increase of manufacturing cost. Furthermore, it also causes an increase of the manufacturing cost of the optical pick up to load the tilt (surface wobble) servo on the optical pick up.
In a case where reproducing is performed from the side of the transferring layer without going through the substrate, a distance between the transferring layer surface and the objective lens of approximately only 0.1 mm is provided. Therefore, in order to prevent the rotating rigid body optical disk from colliding with the objective lens, it is necessary to make the surface wobble small and improve the chucking precision of a chucking device of the optical disk. However, these cause an increase in the cost of the optical disk and the recording or reproducing apparatus.
Hence, the following method has been studied instead of improving the mechanical plane precision of a rigid body optical disk. That is, the optical disk is made flexible, and a guide is provided at a side opposite to the recording and reproducing surface and opposite to the objective lens of the optical pick up. Under this state, the flexible optical disk is put between the objective lens and the guide. By rotating the optical disk, the disk is aerodynamically floated away from (does not come in contact with) the guide in accordance with Bernoulli's equation. It has been also studied that the position of a recording surface against the objective lens can be made stable so that the tilt is made as close to 0 (zero) as possible.
The above discussed principle whereby the surface wobble of the flexible disk is reduced is determined by all of the following elements:    (1) a force from the guide to push down the disk aerodynamically    (2) an elastically repelling force of the disk    (3) a pressure area between the guide and the disk    (4) a centrifugal force of the disk.
The above mentioned element (4) is determined by the rotational speed of the disk. The element (3) is determined by a gap between the guide and the disk. Furthermore, the element (1) and the element (2) are balanced. The rigidity of the disk and an internal loss absorbing the element (1) contribute to the element (2) as parameters. Hence, it is necessary to control these parameters.
As conventional methods to manufacture a substrate of the flexible optical disk, there are three methods, namely, a heat press method, a 2P (photo polymerization) method discussed in Japanese Patent No. 2942430, and a method discussed in the Japanese Laid-Open Patent Application, No. 6-60423. In the heat press method, a thermoplastic resin or a thermosetting resin is applied on a surface of a flexible sheet such as a polycarbonate film and a minute unevenness pattern of a stamper is transferred. After this is heat cured, a transferring layer is deposited. In the 2P (photo polymerization) method, an ultraviolet curing type resin is applied on the surface of the flexible sheet, the minute pattern of the stamper is transferred, and the transferring layer is deposited after ultraviolet curing is performed. In the method discussed in the Japanese Laid-Open Patent Application No. 6-60423, a flexible organic sheet is heated to a temperature higher than the softening point, a stamper is pressed and fixed so as to be transferred, and the sheet and the stamper are peeled after being cooled.
Furthermore, the Japanese Laid-Open Patent Application No. 11-273147 discloses a direct embossing method whereby a transparent film is heated, pressed, and fixed. In the method, the transparent film is heated and then pressed and fixed. Since it is difficult to prevent unevenness of temperature or stress in the surface to a limit, unevenness may be generated in an optical property, mechanical strength, or curvature. On the other hand, the above discussed 2P method has good transferability and is better than other transferring methods on this point.
In the case of the 2P method, the thickness of the flexible optical disk is the sum of thicknesses of the substrate, the film to be transferred, the transferring layer, and the like. Since the film per se is mass-produced industrially, the distribution of the thickness of the film is approximately ±1 μm. However, as shown in FIG. 1, although the normal film has a thickness amplitude of ±1 μm, the thickness change in circumferential direction is drastic and in spike shapes.
In a case where the surface wobble of the disk is made stable by using an air bearing formed between the guide and the rotating disk, the surface wobble can be made stable by the flexibility of the film or the disk.
On the other hand, follow-ability of the focus servo has a limitation at a high frequency area of the surface wobble. Such follow-ability of the focus servo causes an increase in the focus error and makes the precision of recording and reproducing low. This is a big problem when the recording capacity or recording or reproducing speed of the flexible optical disk is made high.
Based on an analysis of a frequency element of the surface wobble of the flexible optical disk, it is found that the high frequency area of the surface wobble largely depends on non-uniformity of the thickness of the film which is a substrate of the flexible optical disk.
An undulation (three-dimensional deformation) of the flexible optical disk is mostly restrained by the air bearing. The surface wobble due to the undulation belongs to a low frequency area of the amplitude of the vibration where the focus servo mechanism may follow. Hence, influence on the generation of the focus error is small. However, the flexible optical disk rotates at a high speed with a constant space between the back surface of the disk and the guide. The non-uniformity of the thickness of the substrate, particularly the non-uniformity of the thickness due to unevenness of the back surface adjacent to the guide, causes the surface wobble of the surface due to the rotation of the disk. At a high frequency area, the surface wobble due to unevenness of the film thickness greatly exceeds the allowable vibration amplitude at which the focus servo mechanism can follow, and therefore the remaining focus error can become large.
Since a cover film required for a blue-ray disk requires a function as an optical transmission layer, it is necessary to make birefringence as small as possible. On the other hand, since a surface recording is performed for a flexible optical disk system, optical property may be free. However, it is necessary to control a property related to a surface property to be as small as possible.